N.Z. MARINE DEPARTMENT-FISHERIE,S BULLETIN No. .į

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I
APPENDIX A
The 1941 Census of the S,elwyn River Run
At the Selw-yn River in Nortl-r Car.rterbury in 1941 an attenrpt."ras macle to ascertain the
t-tunrber of brolvn trout conprising the sparvuing ruu ancl the quantit¡z of eggs produced by
them. The attenlpt was r¡ade after rnan)¡ trout hacl gor.re up beyoncl the proposccl trap-site.
This rvas at the lowest point at rvhich depth of r'vater marle it practicable to rvork-a point
four ancl a half rniles al¡ove Lake Ellesmere ancl about eight nriles belou' u'here tl.!c pernranent
florv of the river began at thc lower eLrcl of some r.rriles o[ cl--¡' ¡i1'g¡ becl Tlre position selccted
was- shortly ,belo.lv u'here traps hacl bcen installecl in most 1,ears siirce 1900 to obtain ova for
hatcl-reries.
A trap anci l-righ protective racli r¡'ere installccl on 6tìr June ancl r¡,'erc r¡aintaiuccl ur-rtil
l4th August. No fish were stripped. llhe scx ancl nulrbers of all frsh entcritrg the trap n'cre
recorcled beforc thcy were allowed to swim iarther upstrcarn, anc1, at weeltl¡' intervals) satrples
of fisl-r were mcasurecl. The rear half c,f thc aclipose lìn of each fish hanclled u'as removecl b¡' a
verticle crrt before the fish was freecl. A runway ran frorn besicle the pouucl up to a srnall
movable panel in the ti¡nbered barrier above. Netting exteuclecl {ror-ir each bauk clown to this
panel from uDstream and contiluecl through to a seconcl poutid to trap kelts returuiug dowtrstreanl.
A door was so placecl that it coulcl closc tl.re top of tl're race or the entrance to the
seconcl pound as lecluired. The erectío¡r alld rnailrtellalrce oI traps ancl the hanclliug of fisl-r
and recorcling of relevant data were cilrriecl out well b1, ì{r F. Davicl, of the Acclirnatization
Society's staff.
The counting and markir-rg of up-runnir-rg frsh proceedecl steadily fronr 6th June until
20th July, by which time more than 10,000 marked frsh hacl been releasecl up, ancl only about
200 kelts (markecl or unnrarkecl) had returnecl clown. It was knowtr that very many fish had
gol1e up before the trap was installecl, ancl that snrall nurnbers hacl beerr spar'vning as early
as mid-April. It became apparent that spent fish u,'ere clistril¡uted throughout the accessible
water above the trap and that they were disinclined to return don''n while the river remained
at norr¡al level. Netting was therefore clone in portions of the river between the trap ar.rcl
the upper limit of pern.Ìanent water at Boyles Forcl at various tinres frorr 21st July until
sth August. The purpose was to determine the ratio of the lcnowu uumber of marked fish in
this area to the unkuown number of nature unmarked fish. This was an application of a
methocl employed by Dahl (1918) h Ì{orwegian lakes manl'years earlier.
The contir-ruecl movements of fisl-r ir-r lessening nnnrl¡ers up iuto the area above the trap
ancl of much smaller numbers of kelts clown through the trap, a{ter tl¡e completion of netting,
involve